Pierce and clinch fasteners of the type disclosed herein have been commercially successful, particularly in conjunction with the automobile industry. For example, U.S. Pat. No. 3,648,747 discloses the general configuration of a fastener and panel assembly similar to that of the present invention. As disclosed in the '747 patent, the fastener (nut) includes a rectangular pilot portion, a panel bearing flange portion on opposed sides of the nut, and reentrant grooves in the flange portions, adjacent the pilot. The nut is flush mounted on a panel by piercing the panel with the nut pilot and deforming the pierced panel edges into the reentrant grooves, preferably without slitting the panel of the outer groove walls. The resultant nut and panel assembly has substantially improved push-out and pull-through strength, as compared with the prior art.
Pierce nuts of this general configuration have also been used to form nut and panel assemblies, wherein the pierce nut pilot is deformed on the nonflanged ends, beneath the panel, to form additional panel bearing abutments, however the panel was simultaneously slit or severed at the outer groove walls when the panel was deformed into reentrant grooves (see for example U.S. Pat. No. 3,439,723). Corner clinching of the nut pilot has also been used commercially to increase the strength of the nut and panel assembly, including pierce nuts of the general configuration disclosed herein (see U.S. Pat. Nos. 2,750,660 and 2,799,188).
Certain nut and panel assembly applications require a low pressure fluid seal between the nut and the panel assembly. For example, in certain automotive applications, water or other fluid must be prevented from seeping through the interface between the nut and the panel to prevent rust from developing or to protect various items from water damage. Generally, automotive specifications require that the seal be capable of containing the fluid and preventing seepage as described herein. Such a low pressure fluid seal between the nut and panel assembly is disclosed in U.S. Pat. No. 4,630,363. The five above-referenced patents are assigned to the assignee of the present application and the disclosures of all such patents are hereby incorporated by reference.
Although the sealed nut and panel assembly apparatus set out in the above-mentioned '363 patent is effective for preventing the transfer of fluid through the nut/panel interface, it was found that in some applications, especially those wherein the nut/panel assembly was subjected to high levels of vibration, the panel was prone to develop stress cracks or otherwise fail due to general fatigue.
Thus, it is an object of this invention to provide an improved die member for use in fabricating fastener/panel assemblies which are resistant to failure due to stress cracking.